A photovoltaic cell is an electronic component which, exposed to light (photons), produces electricity by means of the photovoltaic effect which is at the origin of the phenomenon. The obtained current is related to the incident light power. The photovoltaic cell delivers a DC voltage.
An organic photovoltaic cell is a particular photovoltaic cell. In this case, at least the active layer consists of organic molecules. Consequently, the photovoltaic effect is, for a photovoltaic cell, obtained by means of the properties of semiconducting materials.
By the expression of “semiconductor” is meant a material which has the electrical characteristics of an insulator, but for which the probability that an electron may contribute to an electric current, although small, is non-negligible. In other words, the electric conductivity of a semiconductor is intermediate between the electric conductivity of metals and the electric conductivity of insulators.
The behavior of semiconductors is described by quantum physics by using an approximation with band theory. The approximation with band theory stipulates that an electron in a semiconductor can only assume energy values comprised in continuous intervals called “bands”, more specifically permitted bands, which are separated by other “bands” called forbidden energy bands or forbidden bands.
Two permitted energy bands play a particular role: the last completely filled band, called a “valence band” and the next permitted energy band called a “conduction band.” In a semiconductor, like in an insulator, the valence band and the conduction band are separated by a forbidden band, currently designated by its shorter English equivalent “gap.” The unique difference between a semiconductor and an insulator is the width of this forbidden band, a width which gives each of them its respective properties. The width of the forbidden band is often characterized in energy, this energy corresponding to the energy to be provided to an electron so that the electron passes from the valence band to the conduction band.
A semiconductor is a semiconductor of type p when the semiconductor comprises chemical elements with a valency different from the valency of the atoms of the semiconductor and increasing the concentration of holes in the valence band. Such a semiconductor is also-called a p-doped semiconductor or an electron donor.
A semiconductor is a type n semiconductor when the material comprises chemical elements with a valency different from the valency of the atoms of the semiconductor and increasing the hole concentration in the conduction band. Such a semiconductor is also-called an n-doped semiconductor or electron acceptor.
Further, a semiconductor is considered as organic as soon as the semiconductor comprises at least one bond being part of the group formed by covalent bonds between a carbon atom and a hydrogen atom, covalent bonds between a carbon atom and a nitrogen atom, or further bonds between a carbon atom and an oxygen atom.
Further, in the case of an organic semiconductor, the approximation with band theory is no longer valid but by analogy, molecular orbitals have the same behavior, the HO orbital corresponding to the valence band and the VB orbital to the conduction band. The HO (acronym for “high occupied”) orbital is also designated in English terminology by HOMO (acronym for “highest occupied molecular orbital”) orbital and designates the highest energy molecular orbital occupied by at least one electron. The LU (acronym for “lowest unoccupied”) orbital is also designated in English terminology by LUMO (acronym for “lowest unoccupied molecular orbital”) orbital and designates the lowest energy orbital not occupied by an electron.
Thus, organic semiconductor materials have a forbidden band, the width of which delimits the minimum energy to be provided to an electron for having it pass from a fundamental state to an excited state. The energy for example is provided as light energy. Such a photovoltaic cell is often designated by its acronym OPV for “organic photovoltaic.”
The organic photovoltaic cell thus comprises an active layer. The active layer has a hetero-junction structure obtained by mixing in the bulk, an electron donor material and an electron acceptor material. In this context, an electron donor material is a semiconducting material of type p while an electron acceptor material is a semiconducting material of type n.
A film is a homogenous and continuous layer made in a material or a mixture of materials having a relatively small thickness. By a relatively small thickness is meant a thickness of less than or equal to 500 microns.
A film may also be characterized by its homogeneity and notably the homogeneity of its thickness over the whole of its surface, its aspect (the presence of a de-wetting point, of a drying gradient, and other defects), its roughness and the resolution of the borders of the film (or contours of the film).
Generally, the characteristics of a film depend on several types of factors related to: the technique used for forming the film; the deposited solution (the deposited amount, its wettability on the substrate, its viscosity); to the materials used (their concentrations, their ratios and their solubilities in the solvent, their molar masses).
A film may be formed by means of a wide range of techniques which may be used for forming the different layers of an organic photovoltaic cell such as printing methods (flexography, heliography, heliogravure, offset printing, ink jet printing, etc.) and coating methods (slot-die, curtain coating, knife coating, etc.).
The spin coating technique is the most used for studying the characteristics of organic photovoltaic cells. Centrifugal deposition also-called spin coating is a very widespread method for depositing a thin layer on a planar surface. This method consists of depositing a drop on a rotating plate, the drop then being spread out by centrifugation, in order to form a layer.
The formed film thickness depends on factors related to how the method is applied like angular velocity (the greater it is, the thinner will be the thickness), the acceleration (the greater it is, the thinner will be the thickness) or the operating period (the longer takes the operation, the thinner is the formed film to a lesser extent).
Alternatively, a dip-coating method may be used for forming the film. This technique is based on a principle similar to centrifugal coating. But, in this case, the substrate is soaked in the solution and is removed with a controlled velocity and angle.
Alternatively, another method said to be a “doctor blading” method may be used. According to this method, a razor blade undergoes translation at a defined distance from the substrate with the purpose of spreading out the organic material. With the deposited volume, the translation velocity and the height of the blade, it is possible to define the final thickness of the film.
These manufacturing methods are however not compatible with large scale production which should preferably be carried out by means of continuous processes such as unrolling methods more known under the name of roll-to-roll (noted as R2R in the continuation of the description).
On the other hand, the manufacturing of a photovoltaic module requires observance of a certain number of conditions. An organic photovoltaic module is an assembly comprising at least two distinctive photovoltaic cells close to each other and connected in series or in parallel. The formation of an organic photovoltaic assembly requires deposition of film strip patterns superposed on a substrate, for example strips with a width comprised between 9.5 mm and 13.5 mm have to be separated by an interband area with a width comprised between 0.5 mm and 4.5 mm, the total width of the band and of the interband area being 14 mm. A module consists of the deposition of several layers by various coating or printing methods as illustrated in the Figure which is a sectional view of an organic photovoltaic module.
From among the conditions to be observed, mention may be made of design, structuration, an exact geometry to within one millimeter of the photovoltaic cells at the intermediate area between each cell where the latter are electrically connected with each other, most deposition techniques are unsuitable since they are incompatible with forming strip patterns.
The R2R deposition processes in the liquid state are divided into various categories: the printing methods allow generation of high resolution patterns and the coating methods result from deposition of material over a full width or a solid surface without any pattern.
The methods contemplated for industrial manufacturing of large surface modules are therefore printing techniques such as screen printing, flexography or deposition by an ink jet or pre-metered coating methods such as so-called slot-die, slide coating and curtain coating methods (compatible with more complex coating heads allowing the design of the film strip). Such methods are notably described in the article of Roar Sondergaard et al. entitled “Roll-to-roll fabrication of polymer solar cells” published in the journal, Materials Today of January-February 2012, Volume 15, Number 1-2.
Each of the coating and/or printing methods operates better for specific viscosities, most printing methods requiring high viscosity inks. Perfectly optimized inks may produce sought resolutions and specificities such as the example described in the article of Christoph Brabec et al. entitled “Solution-Processed Organic Solar Cells” published in the journal Materials Research Society Bulletin of July 2008, volume 33.
The goal of our research is the development of low cost, flexible photovoltaic modules by means of roll-to-roll printing and/or coating technologies. In order to attain these goals, the inks having the required properties are to be formulated and the suitable deposition method for this ink is to be selected. The influence of the parameters of the method on the properties of the layer is to be determined. The influence of the drying conditions on the properties of the layer, like the morphology and uniformity, is to be analyzed.
The active layer is coated/printed from a mixture of materials in solution, for which the viscosity, the wettability of the solution on the substrate are parameters to be controlled depending on the materials and on the coating methods used. From among the mixtures in solution usually considered for forming the active layer of a photovoltaic cell, the mixing of poly(3-hexylthiophene) also noted as P3HT with methyl[6,6]-phenyl-C61-butanoate also noted as PCBM is often studied. P3HT is the semiconducting material of type p and PCBM is the semiconducting material of type n.
P3HT is a conjugate polymer which exhibits interesting film-forming properties but polymers are also generally more difficult to synthesize and to purify than small molecules, limiting their appeal in industrial settings and their use at a large scale.
Motivated by the reproducibility of the synthesis of small molecules, research groups conducted studies, for example presented in the article of Bright Walker et al. “Small Molecule Solution-Processed Bulk Heterojunction Solar Cells” published in the journal Chemistry of Materials—Volume 23, Number 3, molecular donor materials allowing high performance, notably strong absorption of light, good photochemical stability. Their relatively compact structures including a few simple synthesis steps make them promising materials for mass production of organic photovoltaic modules.
However, when two low molecular weight materials are deposited in order to form a hetero-junction layer, these materials do not form interpenetrated lattices as observed with polymers. The films obtained from such materials are difficult to produce industrially notably because of their relatively poor solubility or wettability.
Therefore there exists a need for a method for producing an organic film giving the possibility of obtaining an organic film having better properties, for example in terms of homogeneity, roughness and definition of the contours.